In recent years, with remarkable reductions in the size and weight of electronic equipment, there has been a growing demand for reducing the sizes and weights of cells used as a power source for driving the electronic equipment.
To meet such a demand for reductions in size and weight, nonaqueous electrolyte rechargeable cells, typified by lithium ion rechargeable cells, have been developed. In addition, lithium ion capacitors are known as a battery device for an application that requires high-energy density characteristics and high-output characteristics. Sodium ion cells and capacitors are also known, which contain sodium that can be obtained at low cost and is naturally abundant compared with lithium.
For these cells and capacitors, for a variety of purposes, a process of doping an electrode active material with alkali metal in advance (generally referred to as “pre-doping”) is employed. For example, for lithium ion capacitors, pre-doping with lithium is performed for the purpose of decreasing the potential of the negative electrode and increasing energy density. In this case, a method involving in-cell pre-doping with the active material of the negative electrode by using a charge collector having a through-hole is typically employed (see Patent Document 1, for example).
Meanwhile, for lithium ion rechargeable cells, pre-doping is performed for the purpose of reducing the irreversible capacitance of the negative electrode. In this case, in addition to the aforementioned method, a method involving pre-doping of the active material of the negative electrode before assembly of the cell is employed (see Patent Documents 2 and 3, for example). For fabricating sodium-ion battery devices, a method involving pre-doping of the negative electrode with sodium before assembly of a battery device is employed (Patent Document 4).
Patent Document 5 proposes, to suppress decomposition of an electrolyte solution on a negative electrode during initial charging of a rechargeable cell, making a fibrous carbon material, which is used as the negative electrode, in contact with n-butyl lithium in a nonaqueous solvent so that lithium ions can be trapped in the fibrous carbon material.
However, the aforementioned conventional methods are not practical in terms of manufacturing cost and convenience. Besides, in the aforementioned conventional methods, pre-doping is performed on a workpiece formed into an electrode (i.e., an active material layer formed on a charge collector). In this case, an insulating binder is partially bonded to active material particles; therefore, the problem arises that non-uniform progress of pre-doping causes speckles at a so-called solid electrolyte interface (SEI) coating.
On the other hand, Patent Document 6 proposes a method of performing pre-doping with lithium ions quickly, uniformly, and with facility: in the method, pre-doping is performed in such a manner that a material that can be doped with lithium, a lithium metal, and a ball are blended and mixed in the presence of a solvent, using collision and friction with the ball.
Patent Document 7 discloses a method of manufacturing an active material with excellent doping efficiency: in the method, a mixture of an active material and a lithium metal is, for example, stirred or blended in a specific solvent to cause collision between the active material and the lithium metal.
The methods proposed in Patent Documents 6 and 7 do not use an insulating binder or the like and are therefore advantageous in that pre-doping progresses uniformly.